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General Category >> Rubber Side Down! >> New bike fixup, Lazy Mikuni
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Message started by Mark Huppertz on 08/22/17 at 15:41:08

Title: New bike fixup, Lazy Mikuni
Post by Mark Huppertz on 08/22/17 at 15:41:08

Hello!
I have a (new to me) 1998 LS650 that was given to me by a friend who was frustrated because they couldn't get it to run past 3rd gear, no matter how many trips to the shop.
It is orange, but the paint on the sunny side has faded to salmon.
Long story short, I found a bad decompression module, but the most annoying thing was the float was set all the way down so it just filled the case with gas!
Got a replacement non-vacuum petcock, and set the float to the correct level. Then it wouldn't get any fuel in the bowl, no matter how I adjusted the float level.
Tuned out the seat was actually loose and dropping down with the needle (which was probably the issue that got it parked).
I removed the little fork that retains the seat, put it in the vice and put a slight bend in it give some preload.
This worked, fired right up!!!

My main question is this... the Mikuni is the first vacuum slide carb I have ever messed with. It feels really sluggish compared to a direct action cable float, although when the engine is under load it makes a crapload of torque.
But it falls on its face in the higher RPM's though.

Pretty sure it is running lean (I will drive it around and pull the plug for a read).

Are these carb's naturally lazy, or it is mixture the issue do you think?

Kinda wish I could find the same cfm carb in a mechanical version!

Thanks!
Mark

Title: Re: New bike fixup, Lazy Mikuni
Post by norm92de on 08/22/17 at 15:56:54

Hi,
Since it is a new to you bike you need to check that you have the correct jets installed for your altitude. The previous owners could have really messed up the jets whilst trying to make it run.

The jetting can be found on this site. Also I'm sure there will be more help and comments here. I don't think you will have much trouble getting it to run. What you have already found was probably causing all kind of troubles.

They are a pretty trouble free bike when the carb is set up properly.

Title: Re: New bike fixup, Lazy Mikuni
Post by ohiomoto on 08/22/17 at 17:34:01

Also, understand that the throttle opens a butterfly valve and vacuum opens the slide. So you need to make sure the slide and carb are clean, that the diaphragm is not damaged, and it is seated correctly. Sticky slides lead to sluggish performance.

While you're at it, make sure the air jets in the top of the carb are correct (something like 230 and 45). We have seen many where the jets were swapped from top to bottom. Meaning the 52.5 pilot jet and 145 main were found in the top of the carb.

Title: Re: New bike fixup, Lazy Mikuni
Post by Mark Huppertz on 08/22/17 at 18:59:14

Ok, I will pull it apart and check the jet sizes. I would be surprised if they could possible lean it out enough to run lean at 7,000ft elevation.

Maybe the diaphragm is cracked.

I did drill out the brass cap and richened up the idle mixture a turn.

Title: Re: New bike fixup, Lazy Mikuni
Post by batman on 08/23/17 at 07:22:54

If your at 7000 ft you may need to move to smaller jets ,stock pilot jet is a 52.5 ,you may want to try a 50 ,main jet should be 145 which should be OK. The mixture screw should be set for best idle which means in your case you may be turning it in rather than out as the mix tends to be richer at your elevation.(the same reason to use the smaller pilot jet).

Title: Re: New bike fixup, Lazy Mikuni
Post by LANCER on 08/23/17 at 07:24:43

Yes, the CV type carb is sluggish when compared to a direct cable operated carb. There are direct cable operated carbs available if desired.

When you take the carb apart to check for the correct jets, take the opportunity to check/clean the passageways in the carb body. There are a number of them with the smallest being on the little brass tube that extends down from the carb body. It has 3 holes in it, one on the end and 2 on the sides (1 each side) about 1/2-3/4" below the end of the tube.
The pilot system has a bunch of passageways, running from the bowl to the TEV to the choke and into the top, under the diaphragm, and into the carb throat near the end by the cylinder head..
If all of the holes are not clear for air to flow then your carb will not work correctly.
A carb tool, similar to a jetting tool used by gas welders, will also work.
The tool will make the job of cleaning out passageways much easier.

I don't know if we have had anyone living up in the 7000' level before.
It will be interesting to see what you come up with. We can give you pretty good guidance on where to start. There is no doubt info on high altitude jetting in the Tech Section. It has been a while since I looked for that.

Title: Re: New bike fixup, Lazy Mikuni
Post by JLC on 08/23/17 at 15:57:30

My house is at 7400'. Since I removed the plug and adjusted the air/fuel mixture screw, my S40 has no problems with running or backfiring, etc. It is all stock, including carburetor, air filter and exhaust.

Title: Re: New bike fixup, Lazy Mikuni
Post by LANCER on 08/23/17 at 16:11:16

Well, if it is running good then don't mess with it.

Title: Re: New bike fixup, Lazy Mikuni
Post by JLC on 08/23/17 at 19:03:43

That's what I thought, but I am surprised at the number of
Savage/S40 owners who have carburation problems at even lower altitudes.

Title: Re: New bike fixup, Lazy Mikuni
Post by LANCER on 08/24/17 at 02:59:02

The vast majority of folks with carb issues have bikes that have set for extended periods of time or have not been corrected from what the factory did.